Method for producing magnetic recording medium

ABSTRACT

A method for producing a magnetic recording medium by ionicplating having generally uniform magnetic characteristics in every direction comprising generating in a vacuum chamber and in a magnetic field a plasma of the glow discharge of a gas between a negatively-charged magnetic recording medium substrate and a positively-charged evaporative source of a ferromagnetic material and positioning the substrate in a magnetically concentrated zone of the plasma which is concentrated by the magnetic field, whereby the ferromagnetic material is evaporated and deposited on the magnetic recording medium substrate.

United States Patent Shirahata et al.

[75] Inventors: Ryuji Shirahata; Tatsuji Kitamoto;

Mahito Shimizu, all of Kanagawa; Akira Tasaki, Osaka; Masaaki Suzuki,Kanagawa, all of Japan {73] Assignee: Fuji Photo Film Co., Ltd.,

Minami-ashigara, Japan [22] Filed: July 25, 1974 [211 App]. N0.: 491,901

[30] Foreign Application Priority Data July 25, 1973 Japan 48-83836 [52]US. Cl. 204/192; 204/298; 427/39; 427/48; 427/128; 427/132; 340/174 TP[51] Int. Cl. C23C 15/00; B05D 3/14 [58] Field of Search 1l7/93.1 CD, 6D, 93.2, 117/238; 204/192, 298; 250/530; 427/38, 39, 48, 128, 132

[56] References Cited UNITED STATES PATENTS 3,282,815 11/1966 Kay et a1204/192 1 Dec. 30, 1975 3,282,816 11/1966 Kay 204/192 3,306,116 2/1967Maissel et a1. 204/192 3,329,601 7/1967 Mattox 204/298 3,413,141 11/1968Friedman 117/93.2 3,533,836 10/1970 Massengale et a1... 117/2373,616,404 10/1971 Gregory 204/192 Primary Examiner.lohn H. MackAssistant ExaminerAaron Weisstuch Attorney, Agent, or Firm-Sughrue,Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT A method for producing amagnetic recording medium by ionic-plating having generally uniformmagnetic characteristics in every direction comprising generating in avacuum chamber and in a magnetic field a plasma of the glow discharge ofa gas between a negatively-charged magnetic recording medium substrateand a positively-charged evaporative source of a ferromagnetic materialand positioning the substrate in a magnetically concentrated zone of theplasma which is concentrated by the magnetic field, whereby theferromagnetic material is evaporated and deposited on the magneticrecording medium substrate.

10 Claims, N0 Drawings METHOD FOR PRODUCING MAGNETIC RECORDING MEDIUMBACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION This inventionrelates to a method for producing a magnetic recording medium byionic-plating, and, more precisely, to a method for producing a magneticrecording medium having excellent magnetic characteristics, especially ahigh squareness ratio, by ionicplating.

2. DESCRIPTION OF THE PRIOR ART Ferromagnetic thin metal films formed ona substrate by electroplating, non-electrolytic plating, sputtering,vacuum evaporation, ionic-plating or the like have recently becomeworthy of notice as the so-called nonbinder type magnetic recordingmedia in which no binder is used, in place of conventional binder-typemagnetic recording media produced by coating a dispersion of magneticpowders of 'yFe O Co-doped yFe O;,, F 0 CrO or ferromagnetic alloys inan organic binder on a substrate. As one of the essential requisites formagnetic recording media used for high density recording, it isproposed, either theoretically or experimentally, to impart a highcoercive force thereto and to reduce the thickness of the magnetic filmthereof, and improvements in non-binder type magnetic recording mediawhich can more easily be reduced in thickness by a factor of IO thanother coatedtype magnetic recording media and which have a highermagnetic flux saturation are desired and various efforts have heretoforebeen made for the practical use of such advantageous non-binder typemagnetic recording media.

A method of evaporation plating in a glow discharge or a so-calledionic-plating method as disclosed in US. Pat. No. 3,329,601 is onelocated in the intermediate position between the preparation of alloyparticles by low-vacuum evaporation and vacuum evaporation plating, andthis method has the possibility that a magnetic thin film havingsufficient coercive force and squareness ratio suitable for magneticrecording medium can be formed, and so, this method is an interestingmethod. In addition, according tothis method, evaporated metal isionized in the glow discharge field and accelerated by an electric fieldfor adherence on a substrate, and thus, adhesion of the evaporated metalon the substrate is far stronger than the adhesion obtained using otherconventional vacuum evaporation plating methods. Accordingly, themagnetic recording medium produced by this method is suitable for use asa magnetic recording medium which is subjected to severe conditionsunder relative movement with a magnetic head. However, according to theconventional ionic-plating method as described in this US. Pat. No.3,329,601, although improvement of the coercive force can be achieveddue to the pressure of argon gas during the glow discharge, it isdifficult or rather impossible to obtain the high squareness rationecessary for a magnetic recording medium.

SUMMARY OF THE INVENTION An object ofthis invention is to provide anionic-plating method for producing a magnetic recording medium, whichhas markedly improved magnetic characteristics, particularly a highsquareness ratio, and good surface characteristics.

More precisely, this invention provides a method for producing animproved magnetic recording medium by ionic-plating comprisinggenerating in a vacuum chamber and in a magnetic field a plasma of theglow discharge of gas between a negatively-charged magnetic recordingmedium substrate and a positively-charged evaporative source of aferromagnetic material and positioning the substrate in a magneticallyconcentrated zone of the plasma which is concentrated by the magneticfield, whereby the ferromagnetic material is evaporated and deposited onthe magnetic recording medium substrate.

DETAILED DESCRIPTION OF THE INVENTION In formation of magnetic thinfilms for use in memory elements for electronic computers and the likeusing vacuum evaporation coating, plating or the like, a method hasheretofore been practiced where a magnetic field is applied during theformation thereof to induce a uniaxial magnetic anisotropy in themagnetic thin film. It has now been found quite unexpectedly that,according to the method of the present invention, a magnetic thin filmcan be obtained which has improved magnetic characteristics with anextremely high squareness ratio in every direction in the surface of thethin film and also has improved surface characteristics, withoutinducing uniaxial anisotropy in this magnetic thin film. Such phenomenaare considered quite novel, being different from those of otherconventional methods where uniaxial anisotropy is induced in themagnetic film.

More particularly, it has been found that magnetic thin films ofextremely excellent squareness in every direction in the surface thereofcan be produced only when the substrate is positioned in theconcentrated zone of the plasma generated by the applied magnetic field.

These phenomena in the method of the present invention have not as yetbeen completely clarified physically and theoretically, and while notdesiring to be bound to the following theory, it is believed that in themethod of this invention the plasma generated by the glow discharge dueto the magnetic field is concentrated in the vicinity of the substrate,whereby an even film having good surface characteristics and microscopicstructure is plated thereon by ionic-plating and good magneticcharacteristics are imparted to this film.

Representative examples of ferromagnetic substances which can be used inthe present invention are, for example, ferromagnetic metals such asiron, cobalt and nickel, magnetic alloys such as FeCo, FeNi, Co-Ni,FeRh, FeCu, FeAu, CoCu, CoAu, CoY, CoLa, Co-Pr, Co-Gd, CoSm, CoPt,Ni-Cu,'FeCoNi, Mn-Bi, Mn-Sb and MnAl, and ferrite-type magneticsubstances such as Baferrite and Srferrite.

The thickness of the magnetic thin film formed according to the methodof this invention is, in general, in the range of about 0.05 am to L0p.m, preferably 0.1 ,um to 0.4 nm, in view of such essential requisitesthat the film be sufficiently thick that a sufficient output to themagnetic recording medium can be imparted and the film be sufficientlythin that high density recording can be carried out. The strength of themagnetic field used in the present invention rangesfrom abut 50 to 5000oe, practicallypreferably to 2000 oe, on the surface of the substrate.Suitable temperatures which can be used to heat the evaporation sourceof the ferromagnetic material-range from about 1000 to l700C.

Suitable ionic-plating conditions which can be used in the presentinvention are those as describedin the above .mentioned US. Pat. No.3,329,601, and the apparatus for;the method of this invention can easilybe a modification of the apparatus used in the conventionalionic-plating method. More precisely, the degree of vacuum in theapparatus containing an inert gas employed in ionic-plating is, ingeneral, in the range of about 0.00l to 01 Torr, preferably 0.005 to0.05 Torr, and the acceleration voltage potential for the glow dischargeis, in general, about 0.1 to 5 kv, preferable 0.2 to 2.0 kv. The timenecessary for ionic-plating varies, depending upon the processconditions and the thickness of the magnetic thin film desired, but is,in general, about 0.5 to 20 minutes.

Suitable inert gases which can be used in the present support (width: 2inches) was wound around the central region of the magnet between theN-pole and S- pole of the magnet, Co, CoNi and CoCu were plated byionic-plating analogously to the above process. The strength of themagnetic field at the surface of the support was 1000 0e. The luminosityof the plasma was concentrated in the part of the polyethyleneterephthalate support positioned in the central region of the permanentmagnet.

In every case, argon was used for glow discharge, and the conditions ofthe ionic-plating were as follows: degree of vacuum: 0.01 Torr;acceleration voltage: 0.4 kv; time: 6 minutes. The surface of thesubstrate was not pre-cleaned prior to ionic-plating.

The magnetic characteristics of the thus produced magnetic thin filmswere measured and the results obtained are given in the following TableI.

invention are nitrogen gas and noble gases such as helium, neon, argon,krypton, xenon and radon. These can be used alone or as a mixturethereof, if desired.

- According to the method of this invention, it is possible to form aneven magnetic thin film having good adhesion to the substrate thereof byionic-plating, and further, it is possible to form a magnetic thin filmhaving a markedly higher B-H curve squareness ratio than that ofmagnetic films prepared by conventional methods. In high densityrecording with magnetic recording media, the self-demagnetization lossincreases as the wavelengths being recorded decrease, and there fore, ahigher squareness ratio is required for the magnetic recording medium.According to the method of the present invention, it is easy to produceimproved magnetic recording media with these preferred magneticcharacteristics. In addition, according to the present method a magneticthin film having bettersurface characteristics and metallic brilliancethan those of magnetic films produced by conventional ionic-platingmethods can be obtained.

The present invention is explained in greater detail by reference to thefollowing Examples, which, however, are not intended to be interpretedas limiting the scope of the present invention.

EXAMPLE 1 For formation of a ferromagnetic metal thin film theionic-plating apparatus as described in U.S. Pat. No. 3,329,601 wasused, .and films of Co, CoNi and CoCu were formed on a polyethyleneterephthalate support. A'permanent magnet (length: cm) was used as acathode, and the polyethylene terephthalate The samples produced in theplasma have uniform magnetic characteristics as determined in everydirection on the surface of the formed thin film thereof, and thesesamples have a much higher squareness ratio than those produced by theconventional method. In addition, it also is noted that the surface ofthe thin film formed in the plasma generated due to the applied magneticfield had good surface characteristics and remarkable metallicbrilliance.

EXAMPLE 2 Analogously to Example 1, a permanent magnet (length: 20 cm)was used as a cathode, and a polyimide support (width: 2 inches) wasapplied in the regions of a N-pole and S-pole and the central regionbetween the N-pole andthe S-pole of this magnet, and Co, CoFe andCoFe-Cr were plated thereon by ionic-plating.

The strength of the magnetic field in the regions of the N-pole and theS-pole was 700 0e and that in the central region therebetween was 600oe. After the degree of vacuum was adjusted to 10 Torr, helium gas wasintroduced to change the degree of vacuum to 0.01 Torr. Thus,ionic-plating was carried out for 4 minutes where the accelerationvoltage was 1.0 kv. The surface of the substrate was not pre-cleanedprior to the ionic-plating. It is noticed that plasma generated due tothe glow discharge was concentrated in the central region of the magnetduring the ionic-plating, and no luminosity of plasma was observed inthe regions of the N-pole and the S-pole.

Magnetic characteristics of the thus produced magnetic thin films weremeasured and the results obtained are given in the following Table 2.

TABLE 2 Composition of Evapora- Formed in the Central Region live SourceFormed in the N-polc Region of the N-pole and the-Spole (wl'lr) FilmCoercive Squareness Film Coercive squareness Thickness Force RatioThickness Force Ratio (am) (gm) Co 0.20 330 0.61 0.19 350 0.84Co(X0)Fe(20) 0.18 290 0.59 0.19 300 0.80 C0(75)-FC(20)- 0.21 320 0.600.20 300 0.82 0(5) The magnetic characteristics of the samples probe inthe form of a tape, a sheet, a card, a disk or a duced in the S-polewere almost the same as those drum, on which an even magnetic thin filmcan be produced in the N-pole. formed.

The samples produced in the central region with the While the inventionhas been described in detail and concentrated plasma have a uniformsquareness ratio with reference to specific embodiments thereof, it willin every direction in the surface of the thin films be apparent to oneskilled in the art that various formed, and induction of uniaxialanisotropy was not changes and modifications can be made thereinwithobserved therein. lnaddition, the samples produced in 2 outdeparting from the spirit and scope thereof.

the region of the concentrated plasma exhibited a What is claimed is:much higher squareness ratio than those produced in 1. A method forproducing a magnetic recording the region of the N-pole and the S-pole.medium by ionic-plating having generally uniform mag- I neticcharacteristics in every direction comprising gen- EXAMPLE 3 5 eratingin a vacuum chamber and in a magnetic field a Analogously to Example 1,the same ionic-plating plasma of the glow discharge of a gas between anegaapparatus was used and films of Fe, FeNiCo, Co tively-chargedmagnetic recording medium substrate and FeRh were formed on a polyimidesupport. and a positively-charged evaporative source of a ferro- Argonwas used for the glow discharge, and after surmagnetic material andpositioning the substrate in a face cleaning was carried out for 2minutes under a 0 magnetically concentrated zone of the plasma which isdegree of vacuum of 0.01 Torr and a voltage of 2 kv, concentrated by themagnetic field, whereby the ferrothe subsequent ionic-plating wascarried out for 4 minmagnetic material is evaporated and deposited onthe utes under a degree of vacuum of 0.04 Torr and a magnetic recordingmedium substrate. voltage of 1.5 kv. Next, Helmholtz coils forgeneration 2. The method as claimed in claim 1, wherein the of amagnetic field were set so that the substrate holder magnetic field hasa strength ranging from about to containing the polyimide was positionedintermediate 5000 0e. between the two coils. The strength of themagnetic 3. The method as claimed in claim 1, wherein the field on thesurface of the support was adjusted to 160 voltage of the glow dischargeis about 0.1 to 5.0 kv. 0e and ionic-plating was carried out analogouslyto the 4. The method as claimed in claim 1, wherein the above, wherebythe plasma was observed to be concen- 40 degree of vacuum is about 0.001to 0.1 Torr. trated in the vicinity of the surface of the support. 5.The method as claimed in claim 1, wherein the TABLE 3 Composition ofEvapora tive Source Without Applying Magnetic Field With ApplyingMagnetic Field (wt%) Film Coercive Squareness Film Coercive SquarenessThickness Force Ratio Thickness Force Ratio (am) (M) Fe 0.22 180 0.470.25 210 0.73 Fe(40)-Ni(20)- 0.31 510 0.53 0.30 300 0.72 Co(40) (:0 0.32320 0.58 0.55 510 0.75 Fe(97)-Rh(3) 0.23 210 0.51 0.27 210 0.72

Induction of uniaxial anisotropy was not observed in ionic-plating isconducted for about 0.5 to 20 minutes. the above samples produced in themagnetic field, and 6. The method as claimed in claim 1, wherein the gasthese samples were observed to have an improved is chosen from at leastone of the group consisting of higher squareness ratio than the othersamples pronitrogen gas and a noble gas. duced without the magneticfield. 7. The method as claimed in claim 6, wherein said gas Inaddition, the surface of the thin film formed in the is chosen from atleast one of the group consisting of plasma generated due to the appliedmagnetic field was helium, neon, argon, krypton, xenon and radon.

observed to have an excellent metallic brilliance and 8. The .method asclaimed in claim 1, wherein the observation with a scanning typeelectron microscope ionic-plating is continued until the thickness-ofthe confirmed that the surface of the thin film was even. ferromagneticthin film ranges from about 0.05 to 1.0

In the above Examples polyethylene terephthalate pm. and polyimide wereused as the substrate. Other plastic 9. The method as claimed in claim1, wherein the supports such as polyvinyl chloride, cellulose triacetatemagnetic material is at least one ferromagnetic suband polycarbonate aswell as metals such as aluminum stance selected from the groupconsisting of Fe, Co, Ni, and brass can also be used therefor. Thesubstrate can FeCo, FeNi, CoNi, FeRh, Fe-Cu, FeAu,

8 a permanent magnet in the central region between the Co-Sm, CPt,Ni-Cu, Fc-CoNi, MnBi, Mn sb, Ba fcmte and Skierrite' N-pole and S-pole,said permanent magnet acting as a 10. The method as claimed in claim 1,wherein the Cathodemagnetic recording medium substrate is wound around

1. A METHOD FOR PRODUCING A MAGNETIC RECORDING MEDIUM BY IONIC-PLATINGHAVING GENERALLY UNIFORM MAGNETIC CHARACTERISTICS IN EVERY DIRECTIONCOMPRISING GENERATING IN A VACUUM CHAMBER AND IN A MAGNETIC FIELD APLASMA OF THE GLOW DISCHARGE OF A GAS BETWEEN A NEGATIVELY-CHARGEDMAGNETIC RECORDING MEDIUM SUBSTRATE AND A POSITIVELY-CHARGED EVAPORATIVESOURCE OF A FERROMAGNETIC MATERIAL AND POSITIONING THE SUBSTRATE IN AMAGNETICALLY CONCENTRATED ZONE OF THE PLASMA WHICH IS CONCENTRATED BYTHE MAGNETIC FIELD, WHEREBY THE FERROMAGNETIC MATERIAL IS EVAPORATED ANDDEPOSITED ON THE MAGNETIC RECORDING MEDIUM SUBSTRATE.
 2. The method asclaimed in claim 1, wherein the magnetic field has a strength rangingfrom about 50 to 5000 oe.
 3. The method as claimed in claim 1, whereinthe voltage of the glow discharge is about 0.1 to 5.0 kv.
 4. The methodas claimed in claim 1, wherein the degree of vacuum is about 0.001 to0.1 Torr.
 5. The method as claimed in claim 1, wherein the ionic-platingis conducted for about 0.5 to 20 minutes.
 6. The method as claimed inclaim 1, wherein the gas is chosen from at least one of the groupconsisting of nitrogen gas and a noble gas.
 7. The method as claimed inclaim 6, wherein said gas is chosen from at least one of the groupconsisting of helium, neon, argon, krypton, xenon and radon.
 8. Themethod as claimed in claim 1, wherein the ionic-plating is continueduntil the thickness of the ferromagnetic thin film ranges from about0.05 to 1.0 Mu m.
 9. The method as claimed in claim 1, wherein themagnetic material is at least one ferromagnetic substance selected fromthe group consisting of Fe, Co, Ni, Fe-Co, Fe-Ni, Co-Ni, Fe-Rh, Fe-Cu,Fe-Au, Co-Cu, Co-Au, Co-Y, Co-La, Co-Pr, Co-Gd, Co-Sm, Co-Pt, Ni-Cu,Fe-Co-Ni, Mn-Bi, Mn-Sb, Mn-Al, Ba-ferrite and Sr-ferrite.
 10. The methodas claimed in claim 1, wherein the magnetic recording medium substrateis wound around a permanent magnet in the central region between theN-pole and S-pole, said permanent magnet acting as a cathode.